Avoid @generated in our macros

[fingolfin]

Rewrite @wrap, @gapwrap and @gapattribute to not use @generated anymore: we used that as a trick to delay loading of certain objects from GAP to runtime; but this was always brittle and will fail in the future when Julia becomes more effective and aggressive precompilation support.

We replace this with a simpler approach where the GAP objects now really are only loaded at runtime, and store in global const Ref{GapObj} variables. This is in theory slightly less efficient (as now every function call has to check whether the Ref is already assigned), but this is tiny. In practice we still have a noticeable improvement over directly calling the relevant GAP functions. Indeed:

Before this patch:

julia> @btime GAP.Wrappers.IsString(1);
  6.125 ns (0 allocations: 0 bytes)

julia> @btime GAP.Wrappers.String(1);
  154.647 ns (6 allocations: 160 bytes)

After this patch:

julia> @btime GAP.Wrappers.IsString(1);
  9.844 ns (0 allocations: 0 bytes)

julia> @btime GAP.Wrappers.String(1);
  156.197 ns (6 allocations: 160 bytes)

For reference, without the wrappers (same before and after this patch):

julia> @btime GAP.Globals.IsString(1);
  35.624 ns (0 allocations: 0 bytes)

julia> @btime GAP.Globals.String(1);
  182.200 ns (6 allocations: 160 bytes)

This patch is motivated by JuliaLang/julia#36770 and JuliaLang/julia#43990

[codecov]

Codecov Report

Merging #780 (1f05f90) into master (6824be2) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #780   +/-   ##
=======================================
  Coverage   78.18%   78.19%           
=======================================
  Files          46       46           
  Lines        3645     3642    -3     
=======================================
- Hits         2850     2848    -2     
+ Misses        795      794    -1     

Impacted Files	Coverage Δ
src/macros.jl	98.46% <100.00%> (+1.40%)	⬆️
src/GAP.jl	82.17% <0.00%> (ø)